What Makes a Driveway More Than Just a Black Top?
In response to numerous requests from property owners, as well as contractors, for our help in establishing a guideline for paving specifications on residential driveways and small to medium commercial jobs, we would recommend the following in our geographic area1:
Aggregate Base: The Foundation
As with almost any construction project, the foundation is the starting point and its also one of the most important factors when considering pavement design. The aggregate base acts as the foundation for the pavement. The main purpose of the base is to provide a frost barrier that will prevent heaving in the winter, and to provide a stable surface that will support the design load of the pavement under various conditions.
If you are working with a stabilized sub grade that already has a sub base installed, or if the native soil of the site is gravelly in consistency, such as "bank run gravel" and this sub base is well compacted, a 1 to 2 inch layer of processed stone base or recycled aggregate meeting Connecticut Department of Transportation (Conn. D.O.T.) specifications should suffice. As an aside, if the aggregate base meets Conn. D.O.T. specifications it would most likely meet New York State D.O.T. specifications for "Item # 4" aggregate base and most recycled aggregates are acceptable.
If you are working directly on the sub soil that is already at sub grade elevation, and it is a firm, stable surface that is free of organics, we would suggest an aggregate base of not less than 8 to 10 inches to provide an adequate frost barrier and to have enough strength to support the pavement. Soft sub soil conditions may require additional sub soil remediation prior to installing the aggregate base to ensure proper stability or the sub soil may require some means of stabilization be introduced.
The aggregate base material should be placed in thicknesses not exceed 6 inches in any one layer to allow for adequate compaction. If the total required thickness exceeds 6 inches the base should be installed in multiple layers called "lifts." While placing the base material, compaction from a mechanical source such as a vibratory drum roller will be required. A vibratory drum roller, as opposed to a static drum roller, will give the greatest compaction density because the vibrating action of the roller will allow the base material to realign and compact from the bottom up. The vibratory roller should be used where ever possible and a vibratory plate compactor should be used in areas not accessible by the roller. After the compaction process is completed we recommend, when possible, to drive a fully loaded dump truck across the area to observe if there is any deflection in the base. On occasion when thick layers of base are installed, such as 6 inches or more, additional time for settlement may required even after adequate compaction is applied. Sometimes there is no substitute for a little "Mother Nature" such as a good rain fall and some time to dry.
When preparing the base it's important to remember that the finished pavement may not surface drain correctly if the pitch is less than 2% or ¼ inch per foot.So its important to set the pitch correctly with the aggregate base. This also happens to be the same amount of pitch that is found in a buildings drainage or sewer system as mandated by most building codes. What that means is that the slope of the surface should have not less than approximately 2 ½ inches of pitch for every 10 feet of run, because 10 feet equals 120 inches, and 2% of 120 is about 2 ½. This is the minimum standard used in most design criteria that ensures 100% surface drainage and overcomes most small unnoticeable imperfections that occur in the paving process. At times, Architects or Engineers will design a specification that is less than 2%, or at times site conditions just won't allow for enough pitch. Given ideal work conditions, an experienced crew may be able to manage with less pitch, up to a limit, if and when no other solution is possible. But the possible negative effects of this may be that when there is some change to the finished surface profile of the pavement, whether created by traffic or normal wear and tear, the surface will no longer drain the way it did when it was completed. So given the multitude of factors that are commonly found in most residential and commercial sites, it is best to have a little more pitch than a little less.
Pavement Thickness and Installation
When it comes to asphalt paving more is not necessarily better especially if not installed properly. As a rule of thumb, to achieve a compaction density of 90% to 95%, a pavement designed for use in residential driveways and light to medium commercial applications, the finished compacted thickness should not exceed 5 times the largest stone size in the mix. For example, a State of Conn. D.O.T. Class 2, a New York class 6F, or a variation there of, would be used as the finish surface in most driveways. These blends have a maximum nominal aggregate size of between 3/8th inch and ½ inch. This would mean a loose, placed thickness of 2 ½ inches before compaction and a maximum compacted thickness of 2 inches can be used. Generally, the normal amount of compaction is approximately ¼ inch for every loose 1 ¼ inch placed. This of course takes into consideration other factors such as the typical size and weight of most rollers used by contractors doing these types of jobs. A greater loose placed thickness with these types of mix may result in a pavement that is not thoroughly compacted and may fail prematurely because water can get inside the now porous pavement, freeze, and damage the surface.
The often difficult layout of most residential driveways and medium commercial sites, that may have adjoining masonry, structures, and ever changing pitch, presents constraints and challenges in small projects that are normally not found in large scale projects such as road and highway work. To provide proper compaction under those difficult conditions the types of rollers generally being used are a 3 to 5 ton class vibratory roller with 48 inch drums that has a static pound per square rating of 60 psi +/-, along with a smaller, lighter roller and a vibratory plate compactor to remove surface imperfections and finish roll.
Our experience has shown that on small residential driveways that are not large enough to allow a service truck to enter, such as an oil delivery truck or garbage truck, a 2 inch thick single course of pavement will probably be sufficient provided the aggregate base requirements are met as stated in the previous section.
On large driveways, common driveways, private lanes or small to medium commercial jobs where the traffic will be of mixed use including service vehicles such as parcel delivery trucks, oil delivery trucks, garbage trucks, lawn maintenance trucks, etc. a full depth pavement design of not less then 3 ½ inches, placed in 2 lifts such as a 2 inch binder course with a 1 ½ inch top course, may be required and should be considered if the budget can allow for it.
When ever possible, a paving machine should be used to install the pavement, however installing by hand where a paver is not practical or, can not fit into, is acceptable. In any case, an experienced team that has worked together for some period of time, using state of the art equipment that they have been trained and certified on, will produce the best results for the client.
Some other things to consider
As with any consumer based information, we always recommend that the property owner do their homework in selecting any home improvement contractor for any improvement or service. Look at the contractors qualifications and ask a few questions.
Do the contractors you're considering have insurance that is adequate for the project they're about to take on? Are they experienced? Have they been in business for awhile? The U.S. Small Business Administration statistics show that more than 50% of all small businesses fail in the first 3 years and an additional 30% in the next 5 years. That means a warrantee given by an unstable contractor may not be worth the paper it's written on if they're not around to stand behind it should the need arise.
Are their employees background screened? You're about to let total strangers onto your property and near your family or co-workers, so you just can't be too careful. Do they have references? Can you call someone that can vouch for their trustworthiness? A study by the Home Improvement Contractors Association of America found that almost 50% of all consumers that had a home improvement done in the past year would not have the contractor they used back again. That's a flip of a coin odds you will be satisfied or dissatisfied, if you don't choose the right contractor.
Did this potential relationship start with the contractor knocking on your door and begging for work or did you seek them out because you needed them? Did they say they have enough material left over from a job down the road to do your job? If they did then they're probably not very competent or are out right liars, neither of which you should have working on your property.
Do they belong to their industry's professional organizations? These types of organizations are where contractors get the right information, for the right material, and the right way to install it; that's how you get Your Driveway – The Right Way!
The above is meant only as a guide and is taken from such sources as the ABCs of Driveways found on the Asphalt Institute web site, (www.asphaltinstitute.org), studies and research done by the National Asphalt Pavement Association (www.hotmix.org), National Pavement Contractors Association (www.pavementpro.com), generally accepted paving practices and Roccie's Asphalt Paving with their 50+ years of asphalt experience. This should not be used as a pre-engineered specification for a one size fits all application and is relative to the New England geographic area. A qualified, legitimate and experienced contractor can and should be able to determine if any of these suggestions for a given project will be sufficient or if any modification is needed. We can make no warrantee or guarantee, offered or implied, for the statements above. Seek the help of a professional!